This isn't totally beyond the scope of your average do-it-yourselfer, but seriously, don't even attempt it without a good service manual, preferably the OEM manual for your specific motor. It will require several special tools.
The step by step details vary by model, but the procedure most always involves dropping the lower unit, removing the shift rod, propshaft bearing carrier and reverse gear, removing the pinion nut and gear, and pulling out the driveshaft bearing housing and driveshaft, which gives you access to the clutch dog, and forward gear.
good luck.
